1. Field of the Invention
The invention relates to an alarm device, particularly a burglar-alarm device, of the type comprising a first program mechanism at a monitored station and a second program mechanism at a central monitoring station. The program mechanisms are arranged to transmit control signals in accordance with a predetermined signal pattern to a signal comparison circuit at the central monitoring station, which comparison circuit is arranged to generate an alarm signal when the received control signals differ from one another. The first and second program mechanisms have identical electric shift registers with a plurality of binary denominational steps and identical feed-back networks, and each program mechanism is arranged to generate a binary one or zero signal in dependence of the content of the respective shift register, said binary signals constituting discrete control signals.
2. Description of the Prior Art
Alarm devices of this type are difficult to operate, since any disturbance of the equipment at the monitored station causes a difference between the control signals, which results in an alarm being given. One disadvantage inherent with the known devices, is that the signal system is duodirectional, i.e., signals must be transmitted in both directions between the monitored station and the central monitoring station. As a result of this the equipment is both expensive and complicated and requires, inter alia, a duplication of transmitter and receiver, which in turn increases the risk of error and therewith the risk of an alarm being generated. Furthermore, the nature of the signals normally used is such that the signal image becomes distorted during transmission.